Your 780 sits about 13'3". I just measured mine the other day. When I'm hooked up to my van the trailer sits a couple inches higher than the rear lip of the tractor. Hope that helps, I know nothing about pneumatic though.
What kind of pneumatic? The one I pulled was 13’2” tall but it was a 1600cu ft food grade trailer. But if you will be pulling a little sand or cement trailer around your loading and unloading areas will probably be built around a much shorter trailer.